1. Field of the Invention
This invention relates to a so-called dual induction type intake system for an internal combustion engine having an intake passageway for heavy load operation of the engine and an intake passageway for light load operation of the engine.
2. Description of the Prior Art
Generally combustion in internal combustion engines tends to deteriorate during light load operation. As is well known, combustion during light load operation can be improved by enhancing the flow of the intake gas in the combustion chamber to increase the flame speed. For example, there have been developed various intake systems for a reciprocating engine having cylindrical cylinders in which the intake port is offset from the center of the cylinder bore or the intake port is directed substantially in perpendicular to the longitudinal or central axis of the cylinder bore to form a swirl of the intake gas along the peripheral inner surface of the cylinder in the combustion chamber. However, when such a swirl is formed, the intake resistance is increased, which is disadvantageous from the point of increasing the amount of charge. Therefore, it is preferable that the swirl not be formed during heavy load operation in which a good combustion condition can be generally obtained even without forming a swirl.
Thus, there has been proposed in Unexamined Japanese Utility Model Publication No. 55(1980)-4355 an intake system utilizing primary and secondary intake passageways both of which open toward an intake port of the cylinder bore. An intake valve is provided in the intake port to close and open the primary and secondary intake passageways together. However, an independent controllable valve is provided in the secondary intake passageway to selectively close and open it. The open end of the secondary intake passageway toward the intake port is larger than that of the primary intake passageway and is directed substantially in the direction of the central axis of the cylinder bore, while the open end of the primary intake passageway toward the intake port is offset from the center of the cylinder bore and directed in a direction substantially perpendicular to the central axis of the cylinder bore. When the engine is operated under light load, the intake gas is introduced into the combustion chamber only through the primary intake passageway with the secondary intake passageway closed by the controllable valve provided therein, thereby enhancing the swirl formed in the combustion chamber to improve combustion therein. When the engine is operated under heavy load, the controllable valve is opened and the intake gas is introduced into the combustion chamber through both the primary and secondary intake passageways, thereby reducing the intake resistance so that the amount of charge can be easily increased. This system is called the "dual induction system".